Tissue specific cell division inhibitors (chalones) have been well documented in the case of epidermis but evidence for other tissue chalones has been scanty. Our results support the concept of a liver chalone and preliminary studies are consistent with a tumor chalone. The potential application of chalones to the cure of cancer is our ultimate objective. First we plan to develop an assay system in order to aid in obtaining a supply of pure chalone. The immediate objective is to exploit the non-specific nature of chalones in the development of the chick embryo assay. The tissues in chick embryos are in a state of controlled hyperplasia and, with differentiation essentially complete by 15 days of incubation, they should be responsive to chalone. Our results with rat liver extract show this to be true. To shorten assay time, more rapidly determined parameters than enumerating mitoses, such as inhibition of DNA synthesis will be explored. A variety of animal sources will also be explored to establish that best for large scale preparation of crude chalone. As assay techniques are improved, progress can be made on purification. Standard techniques of protein purification will be applied, most of which are in routine use in the laboratory. These include: salt, ethanol or polyethylene glycol precipitation gel filtration, ion exchange chromatography and preparative electrophoresis. At least partial physical characterization (molecular weight, isoelectric point, sedimentation const. etc.) of the chalone molecule will accompany purification. When sufficient liver chalone is available, a greater emphasis will be placed on mechanistic studies using both normal liver and hepatoma-bearing animals. Parallel studies will be performed with other tumors.